xfs_io: fix signed comparison problem in copy_file_range

xfs_io: fix signed comparison problem in copy_file_range

cvtnum() returns a signed long long, so the type of 'len' should be a
signed type so that a user entering a negative length doesn't produce
some huge positive integer.  The negative len check demands it anyway.

Coverity-id: 1435895
Fixes: 25b4549 ("xfs_io: Make copy_range arguments understand *iB values")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>